An active-matrix display device, one of conventional display devices, carries out driving of a display panel, such as a liquid crystal display panel, or an organic EL display panel, by a thin film transistor (“TFT” hereinafter), that is made of an amorphous silicon (amorphous Si; “a-Si” hereinafter) or a polycrystalline silicon (polycrystalline Si; “p-Si” hereinafter) and is formed on a glass substrate.
Particularly common is one with integrated peripheral drivers using p-Si that offers fast operation by its high mobility.
For the system integration of high-performance devices such as an image processor or timing controller, there is a demand for a Si device with better performance.
The need for better performance arises from the insufficient performance of the transistor for making a high-performance Si device, owning to the fact that the mobility is decreased or S coefficient (sub-threshold coefficient) is increased by the presence of a local level in the gap caused by the incomplete crystallinity of the polycrystalline Si, or by the presence of a defect or such a gap local level in the vicinity of a crystal grain boundary.
In light of such a drawback, there has been a technique called SOI (silicon on insulator) in which a monocrystalline Si thin film etc. is bonded with a base substrate. For example, the specification of Japanese Patent No. 3278944 (published on Jul. 22, 1994) describes a method for bonding a base substrate with a substrate previously containing a semiconductor layer. This patent document describes a lamination-type SOI (Silicon On Insulator) semiconductor device in which a base substrate is bonded with a substrate containing a SOI semiconductor layer.
In the technique of the foregoing patent document, the base substrate is bonded with a semiconductor substrate that only contains a semiconductor layer, an element-isolating stage, an insulating layer, and a conductive layer. Meanwhile, there has been known another method in which a base substrate is bonded with a semiconductor substrate that is provided with the whole or the main part of the semiconductor device. Forming the whole or the main part of the semiconductor device on the substrate before bonding the substrate with a base substrate are more advantageous in the micro-fabrication of the monocrystalline Si thin film than forming the whole or the main part of the semiconductor device after transferred onto the base substrate.
For example, the specification of Japanese Patent No. 2743391 (published on Feb. 28, 1990) describes a fabrication method of a semiconductor memory in which a first semiconductor substrate, previously provided with a part of a MIS (Metal Insulator Semiconductor) transistor, is bonded with a second semiconductor substrate in the forming process of a MIS transistor.
Further, as another example, the specification of Japanese Patent No. 3141486 (published on Aug. 13, 1993) describes a semiconductor device including capacitors aligned under the semiconductor layer, wherein the base substrate is bonded with the bottoms of the capacitors via a planarizing layer. In this semiconductor device, non-cell region, i.e., other region than the cell region with the capacitors, is provided with a dummy pattern layer that is the same in thickness as the capacitors, so as to more easily ensure planarization by the planarizing layer, thus increasing bonding strength.
However, the foregoing conventional techniques of Japanese Patents No. 2743391 and No. 3141486 suffer from some difficulties in aligning components (for example, gate electrode etc. of the semiconductor device) in accordance with semiconductor device, after the substrate containing the semiconductor device is transferred to the base substrate.
More specifically, after the substrate having the semiconductor is transferred, the peripheral components should be aligned in accordance with the position of the semiconductor device; however, since the semiconductor substrate is not transparent, the components (gate electrode etc.) under the transferred substrate cannot be seen, thus failing to align them in desired positions.